In a wireless communication network such as a mobile network or a wireless local loop WLL, a base station is installed between a mobile switching center and a mobile terminal of a subscriber, and a wireless signal is exchanged between the base station and the mobile terminal of the subscriber. An antenna is installed in the mobile terminal and the base station for transmitting/receiving the wireless signal.
The antenna of the mobile terminal, as shown in FIG. 1, is conventionally composed in a method where a high frequency signal source 13 is connected between a monopole antenna 12 and a grounded terminal body 11. The monopole antenna 12 applied to the mobile terminal is classified into a whip antenna and a helical antenna.
A whip antenna 15, as in FIG. 2, is designed to have a length of λ/4 in order to maximize its transmission/reception efficiency, if a frequency wavelength is λ. And, the helical antenna 16 has its length of λ/4 in relation to an electric wave and is designed to be twisted in a screw shape in order to shorten the length. The whip antenna 15 has a higher gain than the helical antenna 16, but is designed to be extended because of the appearance due to the length, and designed to be jointly used together with the helical antenna 16. The helical antenna 16 is inserted into and fixed in a housing 14 that is installed at one side of the upper end of the terminal body 11.
In case that the ground-plane of the monopole antenna 12 is a perfect ground-plane, an image is displayed in a grounding opposite direction. The antenna of the mobile terminal is operated like a dipole antenna by the image displayed in the grounding opposite direction and the monopole antenna 12. However, the ground-plane of the mobile terminal is conventionally not formed ideally, thus the ground-plane affects the performance of the antenna of the mobile terminal.
The influence that is caused to the antenna by the ground-plane becomes different in accordance with the length of the mobile terminal. The antenna of the mobile terminal is operated at a maximum performance, when the length L1 of the monopole antenna 12 is λ/4 and the length L2 of the terminal body 11 is λ/4, as in FIG. 1.
The body length L2 of the mobile terminal is conventionally is designed to be λ/4 of the wavelength which is used in a cellular method. Accordingly, the antenna of the mobile terminal operates with the best transmission/reception efficiency when it is operated on a cellular basis because the length of the mobile terminal body 11 is optimized to be λ/4. However, if the mobile terminal is used in a personal communication service PCS method, the wavelength of the electric wave corresponds to half the cellular method in the PCS of which the usage frequency is approximately two folds higher than the cellular method, thus the length of the mobile terminal body 11 is λ/2. Because of this, the current distributed in the mobile terminal body 11 becomes relatively larger than the current radiated in the monopole antenna 12. As a result, in the PCS, as in FIG. 3, an antenna radiation pattern 20 is tilted downward, i.e., toward the terminal body 11. Accordingly, if the mobile terminal designed on the basis of the cellular method is applied to the PCS method, the antenna radiation pattern 20 is reduced in the direction of −90° to +90° as in FIG. 3. This phenomenon acts as a cause that reduces the transmission/reception efficiency of the mobile terminal when considering that the base station antenna is located on the top of the terminal.